Femtocells are small, low-power, indoor cellular base stations designed for residential deployment. They provide better network coverage and capacity than that available in such environments from the overlying macrocellular network. In addition, femtocells use a broadband connection to receive data from and send data back to the operator's network (known as “backhaul”).
Conventionally, femtocell devices are configured to treat users in one of two ways. In the first configuration, the femtocell device acts as an extension to the existing macrocellular network and provides enhanced network coverage and capacity for all users in range of the femtocell device. In a residential context, this means that the femtocell device will provide access to the network for those resident in that location, as well as for neighbours and passers-by if the macrocellular coverage is poor. In this configuration, all users are considered to be part of an open subscriber group, and the use of the femtocell device is not restricted to any particular set of users. Any interference between transmissions from the femtocell device and the overlying macrocellular network will be mitigated by the ability for users to handoff between the femtocell device and the macrocellular network. However, in this configuration, the femtocell device will use the (otherwise private) broadband connection of the femtocell device owner to provide backhaul for public access to the network, and it is not easy to provide users with a reliable quality of service (QOS).
In the second configuration, the femtocell device restricts access to the network to a defined set of subscribers (a closed subscriber group), which results in a better quality of service (QOS) and higher data bandwidth for the subscribers in the closed group. This defined set of subscribers will usually be agreed between the femtocell owner and the network operator (for example those usually resident at the address where the femtocell device is located). However, in this configuration, transmissions from the femtocell device to users in the closed subscriber group can interfere with transmissions from the overlying macrocellular network to other users (not in the closed subscriber group) in the vicinity of the femtocell device. For example, if the femtocell device is using the same channel as the surrounding macrocellular network cell, there could be interference between the two.